Compression rings of various cross-sectional configurations (e.g. rectangular & keystone) have heretofore been utilized in various types of internal combustion engines. When utilized in heavy-duty diesel automotive applications, it has been found that rectangular compression rings have a propensity to stick in the piston head groove after a relatively short period of time. The sticking problem occurs by reason of carbon encrustations forming within the clearance between a side surface of the ring and an adjacent side wall of the groove. When the carbon encrustations have grown to a size wherein there is no longer clearance, an axial side force is exerted within the groove. By having a large clearance between the ring and the groove side wall, serious build up of carbon encrustations can be delayed to a certain extent; however, this advantage is outweighed by the fact that the incidents of ring breakage are directly proportional to the increase in the size of the side clearance between the ring and groove side wall.
With keystone rings the sticking problem is substantially avoided where the tangent of the keystone side angle of the ring is greater than the coefficient of friction between the ring and the groove side wall. When, however, a conventional keystone shaped ring is being utilized it is necessary that a complemental precision ground keystone groove be formed in the piston head. Such a machining operation is more complex and costly than forming a conventional rectangular groove in the piston head.
The capability presently exists to produce at less cost much tighter width tolerances on both conventional rectangular shaped rings and grooves than on the keystone shaped counterparts. Furthermore, in operation the side clearance of a rectangular shaped ring will normally increase due to ring side wear and/or groove side wall wear, whereas, with a keystone ring there are normally four conditions which will increase such clearance; namely, ring side wear, groove side wall wear, wear at the outside diameter of the ring, and/or radial piston head motion, sometimes referred to as "slap."